1. Field of the Invention
The present invention relates to a method for communicating across an electrical isolation barrier, and in particular, to a method and apparatus utilizing Peltier heating and the Seebeck Effect to permit communication of a thermal signal across an electrical isolation barrier.
2. Description of the Related Art
Electrical isolation boundaries are generally employed either to eliminate interference between electrical circuits, or to provide a current barrier for safety purposes. One example of an electrical isolation barrier is found in a power supply which runs off of a wall plug, but which has outputs electrically isolated from the wall plug itself. Another example of utilization of an electrical isolation barrier is found in the interface to a telephone system. Voice, data, and supervisory functions requiring a power supply to operate must be able to transmit signals between telephone lines and telephone or computing equipment while maintaining at least 1000 V.sub.RMS electrical isolation between the user and the power supply. This isolation ensures the safety of the user pursuant to FCC regulations.
A variety of methods have previously been employed to permit communication between systems separated by an electrical isolation barrier. All of these methods involve conversion of electricity to some other form of energy, which is transmitted across the boundary and then reconstituted in the form of an electrical signal.
Optoisolators convert an electrical signal into radiant energy, and then transmit the radiant energy across the electrical isolation barrier. Optoisolators consist of a light emitter and a light detector sealed together in a light-tight package. The intensity of the light emitted varies according to the amplitude of the electrical signal applied to the light emitter. The light detector receives the emitted light and in turn produces an electrical output having an amplitude corresponding to the intensity of the light received. The emitter and detector are manufactured as two separate elements, and are separated by an electrical isolation barrier in the form of a transparent or translucent electrically insulating material such as glass or plastic.
Another manner of communicating across an electrical isolation barrier is by the use of a transformer. Transformers convert an electrical signal into a magnetic field using a first electrically conducting coil. A second electrically conducting coil is then exposed to the magnetic field, either by virtue of physical proximity to the first coil or by directing the magnetic field using a magnetic material or "core." The second coil produces an electrical output in response to changing intensity of the magnetic field.
While optoisolators and transformers function adequately in many applications, these structures present a number of disadvantages. Most importantly, optoisolators and transformers cannot readily be integrated into an integrated circuit without utilizing expensive processing techniques. This is because complex and specialized structures are required to perform light emission and detection functions, or to generate and detect magnetic fields. As a result, optoisolators and transformers are generally only available in discrete form, and thus present economic and volumetric penalties to system designers.
Therefore, there is a need in the art for a device which enables systems to effectively communicate across an electrical isolation barrier but which avoids the complexity and cost of existing coupler technologies.
U.S. Pat. No. 4,757,528 to Falater et al. ("the Falater Patent") describes a coupler that utilizes transmission of a thermal signal in the form of Joule's heat across an electrical isolation barrier. Specifically, the Falater Patent describes the application of current to a resistor positioned on one side of the boundary. Passage of current through the resistor generates a thermal signal in the form of a temperature increase in the resistor and the surrounding material. This temperature increase may propagate across the electrical isolation barrier and be detected by changes in base-emitter voltage of a bipolar transistor positioned on the other side of the boundary.
The thermal coupler disclosed by the Falater Patent offers the advantage of utilizing components such as resistors and bipolar transistors that are readily incorporated in an integrated circuit. However, the Falater Patent suffers from a major disadvantage in that a thermal signal may be transmitted across the boundary only in the form of a temperature increase. Thus, the bandwidth of communication across the electrical isolation barrier is necessarily constrained by upper temperature limits inherent in the system. These temperature limits may be dictated by such factors as the composition of the resistor, the composition of the electrical isolation barrier, the intended use of the device, and the heat tolerance of other semiconducting structures present in the device.
Moreover, the effective bandwidth available to the device described in the Falater Patent would be expected to shrink over time. This is because repeated transmission of signals across the boundary would cause the resistor and the surrounding material to heat up, thereby narrowing the range of temperature changes detectable over an increasingly hot background.
Therefore, there is a need in the art for a device that enables a thermal signal to be effectively communicated across an electrical isolation barrier without the limitations in bandwidth present in existing designs.